1. Field of the Invention
The invention relates generally to memory management in a multiprocessor system and, more particularly, to enhancing a lock acquisition mechanism.
2. Description of the Related Art
In a large symmetrical multi-processor system, lock acquisition is frequently used to synchronize access to data structures. Systems that run with producer-consumer types of applications have to make sure that the produced data is globally visible before signalling to the consumers so that they can access the produced data structure. Usually, the producer tries to acquire a lock using a lock-load instruction and verify on a lock-word value. Once the producer application has acquired the lock, the producer application is the owner of the data structure until it releases the lock. The consumer will have to wait for the lock to be released before accessing the data structure.
When attempting to acquire a lock, software “spins” or loops on an atomic update sequence that executes the lock load instruction and compares the data with a software specific definition indicating “lock_free.” If the value is “not free,” a branch back to lock load instruction is taken to restart the sequence. If the value does indicate free, the loop is exited and a conditional lock_store instruction is used to update the lock word to “lock taken.” The lock store fails if the processor attempting to acquire the lock no longer holds the reservation made at lock load time. If this lock store fails, software again restarts the loop beginning with the lock load instruction. This spin loop of continually reading and re-reading the lock word when the lock is taken causes the same data to be retrieved out of cache over and over while the lock is taken by another processing element. Accessing the cache array to get the same data and send it again and again, while the lock is taken by another processor, is power consuming, is wasteful of cache access cycles (in cases of shared caches) and could create system live-lock in a large configuration system.
Therefore, a need exists for a system and method for saving power and preventing a potential live-lock situation.